Nonaqueous electrolyte secondary batteries employing nonaqueous electrolytes are widely utilized today as secondary batteries with high energy density. In a nonaqueous electrolyte secondary battery, for example, transfer of lithium ions between a positive electrode and a negative electrode causes charges and discharges.
In general, a lithium transition metal composite oxide having a layered structure such as lithium-nickel oxide (LiNiO2), lithium-cobalt oxide (LiCoO2), or LiNi1/3Co1/3Mn1/3O2 is employed as the positive electrode, and a carbon material capable of storing and releasing lithium, a lithium metal, a lithium alloy or the like is employed as the negative electrode in such a nonaqueous electrolyte secondary battery.
The nonaqueous electrolyte produced by dissolving electrolyte salt such as lithium tetrafluoroborate (LiBF4) or lithium hexafluorophosphate (LiPF6) in an organic solvent such as ethylene carbonate or diethyl carbonate is used.
While such nonaqueous electrolyte secondary batteries have recently been used as power sources for mobile equipment and so on, it is desired to develop nonaqueous electrolyte secondary batteries capable of providing higher energy density with increasing power consumption due to expansion in functionality of the mobile equipment.
Increased capacity of positive electrode materials is required to obtain nonaqueous electrolyte secondary batteries with high energy density; therefore, there have recently been more interest in layered compounds as the positive electrode materials. Positive electrode active materials such as Li2MnO3 and Li1.2Mn0.6Ni0.2O2 that is a solid solution of Li2MnO3 have been studied in order to obtain the positive electrode materials with increased capacity.
Meanwhile, it is known that a sodium transition metal oxide, which is a layered compound, is relatively easy to be synthesized, while many of lithium transition metal oxides, which are layered compounds, are difficult to be synthesized (see Patent Document 1, for example). Particularly, Na0.7CoO2 and NaCo1/2Mn1/2O2 subjected to ion exchange of sodium for lithium can be utilized as the positive electrode active materials for lithium ion batteries.
Therefore, studies on a synthesis method of a sodium transition metal oxide and an ion exchange method, which is a chemical technique, have recently been active.
[Patent Document 1] JP 2002-220231 A